One small lake primarily engulfing this land? (4)
I believe the answer is:
isle
'this land?' is the definition.
(isle is a kind of land)
'one small lake primarily engulfing' is the wordplay.
'one' becomes 'I' (Roman numeral).
'small' becomes 's' (abbreviation - e.g. clothes size).
'lake' becomes 'l' (geographical abbreviation).
'primarily' says to take the initial letters.
The first letter of 'engulfing' is 'e'.
'i'+'s'+'l'+'e'='ISLE'
(Other definitions for isle that I've seen before include "small piece of land" , "Anglesey, say" , "Dogs, perhaps" , "Ireland, perhaps" , "Small mass of land in the sea" .)